The eruption of teeth from their place of development within bone to their place of function within the oral cavity is a complicated process. Recent experiments have shown that the periodontal ligament, which attaches the tooth to alveolar bone, is capable of providing a continuing eruptive force once the bone over the tooth bud has been removed. Other experiments suggest that formation of the eruptive path in bone by resorption is coordinated with, but not dependent upon, tooth eruption. Little is known of the role of bone resorption in tooth eruption or the factors involved in this localized resorption of alveolar bone. The relationships between tooth eruption and bone resorption will be examined in an experimental animal, the ia rat, in which bone resorption is reduced and teeth do not erupt. The cells responsible for bone resorption, the osteoclasts, can make but not release membrane- bound packages of acid phosphatase, an enzyme associated with bone removal. Factors reducing bone resorption will be studied by examining the fine structure of ia osteoclasts for changes in cellular membranes and by quantitative assay of acid phosphatase and other enzymes in isolated osteoclasts. Attempts will be made to increase resorption in vivo by injections of vitamin A (a substance that releases acid phosphatase from cells) and by parabiotic union with a normal littermate. The increased resorption will be correlated with the development and eruption of the teeth. This study is expected to elucidate the importance of the osteoclast in tooth eruption and to explore the mechanisms by which the function of osteoclasts is reduced in ia rats. A method to reduce bone resorption would have immediate clinical application to the prevention and treatment of alveolar bone loss in periodontal disease.